List Name

Grades

Hurricanes: The Basics

Grades

3–5,
6–8

In this photo of a hurricane from space, the eye of the storm shows
up blue in the middle of circling clouds. (Photo: NOAA)

What is it?

A hurricane is a swirling storm measuring 60-1,000 miles in diameter
that forms over warm ocean waters. Hurricanes start life as a cluster
of strong thunderstorms moving across the ocean, called a tropical
disturbance or tropical wave. Atmospheric conditions must be
just right to turn a tropical wave into a hurricane  less than 5% of
them ever become full-blown hurricanes.

A tropical wave that begins spinning around a center of low pressure
is called a tropical depression. Tropical depressions have maximum
sustained wind speeds of less than 40 mph at the ocean's surface.
When the maximum winds reach 40 mph or greater, the storm changes into
a tropical storm, and it's given a name. Once the maximum winds
reach 74 mph or greater, the storm becomes a hurricane.

Each hurricane has an eye of calm winds and low pressure, surrounded
by an eyewall of intense thunderstorms with high winds and heavy
rain. Spiral bands of intense thunderstorms spiral into the eyewall
of the hurricane from the outer parts of the storm.

The generic name for a hurricane is tropical cyclone. These storms
are called typhoons when they occur in the western Pacific Ocean,
and cyclones in the Indian and southern Pacific Oceans.

Most
hurricanes happen between June 1 and November 30. Warmer water temperatures
are one reason there are more hurricanes in the summer.